Method of heat treating a radioactive surface

ABSTRACT

In a method of treating a surface 12 of an object 10 contaminated with radionuclides 14, a laser source 16 is directed at the surface 12 to apply a local area 18 of intense heat to the surface 12. The laser source 16 is arranged to pass in a raster manner to cause local melting of the surface 12, surface 12 subsequently solidifying and fixing the radionuclides 14 therein. At least one layer of a coating material be applied before or after the application of the intense heat to fix and seal the radionuclides on or in the object.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of treating a surface, and moreparticularly a surface contaminated with radionuclides.

2. Discussion of the Prior Art

In the nuclear industry, surfaces of objects including mechanicalcomponents and constructional features may become contaminated withradionuclides such as cobalt-60, caesium-137 or strontium-90, orradioactive compounds such as PuO₂ or UO₂. Current practices fortreating these surfaces include the use of chemical reagents, andabrasive jets. However, the contaminating radionuclides may penetratedeeply into the surface portion of the components or features and maypresent difficulties in being removed by these known surface treatments.

A number of alternative surface treatments have been tried by others.One such treatment is described in European patent specification numberEP 91646 A1 which discloses a method of removing a radioactive metaloxide from the surface of a radioactive component by means of a laserbeam directed at the surface. In UK patent specification number GB2242060 A a concrete surface contaminated with tritium is treated byirradiating the surface with microwaves in order to vaporise water fromthe surface thereby removing tritium. German patent specification numberDE 3500750 A discloses a method for removing radioactively contaminatedsurface layers of concrete from a reinforced concrete structure byinductively heating the reinforcing bars within the structure. In afurther method, described in Japanese patent specification number JP3002595 A, a radioactively contaminated concrete surface is removed byirradiating the surface with microwave radiation.

In all of these alternative treatments radioactive contamination isremoved from a surface or else the contaminated surface is itselfremoved. Because of the nature of these treatments, the contaminationbecomes airborne thus necessitating downstream processing and leading tofurther complications and expense.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method oftreating a surface contaminated with radionuclides, the methodcomprising passing a local area of intense heat across the surface so asto fix or seal the radionuclides therein.

As stated previously, the aforementioned alternative treatments are usedto remove contamination from a surface or to remove a surface layercontaining contamination. None of these aforementioned treatmentsprovide a method which achieves fixing or sealing of the contaminationto a surface as is provided by the present invention. The presentinvention allows simpler and cheaper treatment.

Desirably, in the present invention, the intense heat has an energylevel of at least 150 W/cm². Preferably, the intense heat is applied bya laser source, or from a laser source through a fibre optic cable.

The local area of intense heat may be passed, eg in an x-y rasterfashion across the surface by moving the object defining the surfaceand/or by moving a source of the intense heat. A relatively largetreatment area may be achieved by overlapping movement of the objectand/or the source of the intense heat.

The contaminated surface may comprise a layer applied to an object, forexample a paint, or a plastics coating such as an epoxy layer.

At least one layer of a coating material may be applied before or afterthe application of the intense heat to fix and seal the radionuclides onor in the object by melting the coating material and forming a bond ofthe coating material to a substrate, or by forming a fused layercomprising the coating material and said substrate material. Examples ofcoating materials include glass, metal, ceramics, pozzolana andchamotte, or a mixture thereof. A further application of intense heatmay be necessary to bond the coating to the surface.

In another application of the invention to a metal surface, the localarea of intense heat causes local melting of the metal at the surfacewhich subsequently solidifies as the local area of intense heat passesacross the surface. The melting and re-solidification at the surfacefixes the radionuclides in the metal and may repair local faults at thesurface such as porosity or cracks.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described by way of example only withreference to the accompanying drawings in which:

FIG. 1 shows a side sectional representation of the invention applied toa metal object;

FIG. 2 shows a view in the direction of arrow A of FIG. 1;

FIG. 3 shows a side sectional representation of an embodiment of theinvention applied to a concrete object;

FIG. 4 shows a side sectional representation of an alternativeapplication of the invention to a concrete object, and

FIG. 5 shows a side sectional representation of a further alternativeapplication of the invention.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a portion of a steel object 10 is shown having asurface 12 with an internal layer 13 in which radionuclides 14 areembedded. A laser source 16 is shown directed at the surface 12 to applya local area 18 of intense heat to the surface 12. The laser source 16as shown in FIG. 2 is arranged to pass in a raster manner, as shown bythe arrows, across the surface 12 to pass the local area 18 of intenseheat across the surface 12.

In operation, the local area 18 of intense heat applied by the lasersource 16 is arranged to cause local melting at the surface 12 withoutvaporization thereof, the molten surface 12 subsequently solidifying andfixing the radionuclides 14 therein as the laser source 16 passes acrossthe surface 12.

In an alternative application of the invention shown in FIG. 3 to aconcrete object 50 having a surface 52 contaminated with radionuclides(not shown), a layer 54 of a sealant is applied to the surface 52 and ismelted by a local area 55 of intense heat applied by a laser source 56so as to fix the radionuclides to the surface 52. Suitable sealantsinclude: an inorganic paste such as water glass, metal powder, ceramicpowder, glass powder, pozzolana and chamotte, or a mixture thereof, andmay be applied by conventional techniques such as spraying. Theapplication of pozzolana and chamotte to a concrete surface causes areaction with free lime at elevated temperatures. This generates aceramic bond of the coating to the concrete surface, and leaves a glassysubstantially poreless coating after application of the intense heat.More than one such layer 54 may be applied.

The invention may be performed by alternative heat sources such as:flame, plasma ion, ultrasonic energy, microwaves, and induction heating,for example to melt the layer 54. Suitable laser sources include: a CO₂laser, a Nd-YAG laser, an excimer laser

, or a semi-conductor laser. A neodymium-yttrium aluminium garnet(Nd-YAG) laser source is preferred since the radiation therefrom may betransmitted through a fibre optic cable. Such a cable is readily movableto facilitate movement of the transmitted local area of intense heatfrom the laser source across the surface.

If desired the use of an appropriate sealant layer 54 nay be applied tonon-concrete surfaces, eg steel.

For most applications of the invention, a local area of intense heat ofat least 150 W/cm² is preferred.

It will be understood that instead of or as well as moving the lasersource or the fibre optic cable in the afore-described applications ofthe invention, the object having the contaminated surface may be movedto pass the local area of intense heat across the surface.

Referring to FIG. 4, a portion of concrete object 60 is shown having asurface 62 contaminated with radionuclides (not shown). A first layer 64of cementitious material is applied to the surface 62, and is set on thesurface 62 with the assistance of heat from a laser source 66 arrangedto be traversed across the first layer 64, it is soaked with water forabout one minute from a water source 68 to reverse the dehydration oflime in the first layer 64, and allowed to reset for more than twentyfour hours. A second layer 70 of cementitious material similar to thefirst layer 64 is applied to the first layer 64, and heat from the lasersource 66 is then traversed across the second layer 70 in `x-y` rastermanner to set the second layer 70 and produce a vitreous surface 72.

The cementitious material for the first layer 64 preferably comprises amixture in optimum proportions of:

Chamotte--70%

Pozzolana--10%

industrial water glass--20%

and the second layer 70 preferably comprises a mixture in optimumproportions of:

Pozzolana--40%

Pozzolan--35%

Chamotte--20%

industrial water glass--5%

water

Such a cementitious material should provide sufficient silicate contentfor the formation of glass in the second layer 70 after heating by thelaser source 66, although if desired the first layer 64 and the secondlayer 70 may have compositions that differ from each other.

It is an advantage if the direction of traverse of the laser source 66on the second layer 70 is perpendicular to the direction of traverse ofthe laser source 66 on the first layer 64, since this should lead to asmoother surface with improved impact resistance of the second layer 70.

Some advantage might be gained in impact resistance of the second layer70 by adding small amounts of granite powder, or metal powders such asstainless steel to the cementitious mixture. Small amounts of zincpowder in the mixture should also improve the smoothness of the layers64, 70.

For some applications, a thickness of each layer 64, 70 of between 0.5mm and 0.8 mm should be satisfactory.

Suitable lasers include a 2 kW Electrox CO₂ laser, and a 400 W LumonicsNd-YAG laser. The Nd-YAG laser can be transmitted through opticalfibres. A laser beam of spot size between 4 to 8 mm diameter may beused. If desired the surface to be heated by the laser source 66 may beprotected by an inert shroud gas such as nitrogen or Argon.

Referring now to FIG. 5, a portion of a concrete object 80 is shownhaving a surface 82 contaminated with radionuclides (not shown). A thicklayer 84 (eg <5 mm) of cementitious material is applied to the surface82, and heat from a laser source 86 then applied to the layer 84 to forma vitreous coating (1 mm) at the surface 88 of the layer 84. The layer84 preferably comprises a mixture of:

Chamotte

sand/granite

Pozzolana (small amounts)

industrial water glass

water

Use of a relatively high percentage of Pozzolana/Pozzolan at the top ofthe layer 84 assists in the formation of the vitreous coating at thesurface 88.

A laser source 86 similar to the laser source 66 may be used. Thethickness of the layer 84 inhibits heat from the laser source 86reaching the surface 82 at a temperature high enough (500° C.) to causesubstantial dehydration of free lime in the layer 84 at the surface 82.

Before the layer 84 is applied to the surface 82, an initial heattreatment may be applied to the surface 82 by the laser source 86.

We claim:
 1. A non-contact method of treating a surface of an objectcontaminated with radionuclides, the method comprising the stepsof:applying at least one layer of a coating material to said surface;and passing a local area of intense heat across the coating materialthereby providing a vitreous coating over the surface and fixing orsealing the radionuclides therein.
 2. A method as claimed in claim 1,wherein the local area of intense heat has an energy level of at least150 W/cm².
 3. A method as claimed in claim 2, wherein the intense heatis provided from a source comprising a laser means.
 4. A method asclaimed in claim 3, wherein the laser means includes a fibre optic cablethrough which the intense heat from the laser is applied.
 5. A method asclaimed in claim 3, wherein the laser means comprises aneodymium-yttrium aluminium garnet laser.
 6. A method as claimed inclaim 1, wherein the intense heat is passed across the surface by movingthe object relative to the source of the intense heat.
 7. A method asclaimed in claim 6, wherein the source of the intense heat and theobject are moved in overlapping manner.
 8. A method as claimed in claim1, wherein the surface comprises a metal, and the intense heat is suchas to melt the surface.
 9. A method as claimed in claim 1 and wherein afurther layer of a coating material is applied to the surface after theapplication of the intense heat.
 10. A method as claimed in claim 1,wherein the contaminated surface is a cementitious surface.
 11. A methodas claimed in claim 1, wherein the contaminated surface is a metallicsurface.
 12. A method as claimed in claim 1, wherein the coatingcomprises at least one of a refractory material, a cementitiousmaterial, a metal powder, a ceramic, and a mixture thereof.
 13. A methodas claimed in claim 3, wherein the laser means comprises one of a Nd-YAGlaser, a CO₂ laser, an eximer laser and a semiconductor laser.